Firefighters conduct a search and rescue on Thursday of an apartment building destroyed by an explosion at a fertilizer plant in West, Texas, late Wednesday. The massive explosion at the plant killed as many as 15 people and injured more than 160.

The remains of a fertilizer plant burn after an explosion at the plant in the town of West, Texas.

Mike StoneReuters /Landov

After a fire and explosion at a fertilizer plant in West, Texas, that killed as many as 15 people and injured more than 160 others, rescue workers on Thursday are still sifting through the smoldering rubble hoping to find survivors.

Here's what we know at this hour:

- The explosion occurred about 8 p.m. with the force of a small earthquake, leveling a four-block radius around the plant, including dozens of homes.

- Anywhere from 5 to 15 people, including some first responders, have been killed. Others remain missing. More than 160 are injured, The Associated Press reports.

- Police say there is no immediate indication that the blast was anything other than an industrial accident.

We will be updating the story as the day goes along.

Update at 7:59 p.m. ET. Search And Rescue Continues:

The Dallas Morning News quotes West Mayor Tommy Muska as saying that search and rescue teams have combed through "80 percent of the devastated areas, including a nursing home and 50-unit apartment complex, have been searched so far. Teams have found eight to 10 bodies and expect to find at least a half dozen more at the West Fertilizer Co. facility when they can search those grounds."

UPDATE at 4:45 p.m. ET:

Sgt. Jason Reyes of the Texas State Rangers, declined to give any new casualty figures, saying it was still too soon.

Chief Deputy of the McLennan Co. Sheriff's Department, Matt Cawthorn that the area where the explosion took place is "highly populated ... It is devastated."

He said the cause of the initial fire has not yet been determined.

UPDATE at 4:30 p.m. ET:

NPR's Daniel Zwerdling reports that there's nothing in the history of the fertilizer plant that could have predicted Wednesday's disaster. Even so, it's record was not spotless:

"First, the Texas Commission on Environmental Quality got a complaint in 2006 — the person who filed it said the ammonia smell around the plant was 'very bad.' State regulators inspected and they found that the company didn't have a proper permit. Then the federal EPA inspected and they found that the company didn't have a formal written maintenance plan. It had failed to consider potential hazards properly and plan how to deal with them," Zwerdling says.

UPDATE at 12:55 p.m. ET: Governor Perry Makes Disaster Declaration

Gov. Rick Perry has declared McLennan County, where West is located, a disaster.

"Last night was truly a nightmare scenario for that community," he said at a news conference.

"This tragedy has most likely hit every family, has touched nearly everyone in that town," he said.

He said the scene of the explosion was still an "active search and rescue mission" and that the state would offer resources as long as they were requested and needed.

UPDATE at 12:15 p.m. ET: Anhydrous Ammonia Vs. Ammonium Nitrate

NPR science reporters Christopher Joyce and Dan Charles have weighed in to help clarify the difference between these two chemical compounds:

Anhydrous ammonia is harmful to skin and very dangerous to breathe, but it's not the same as ammonium nitrate, used in the explosive at the 1995 Oklahoma City bombing, they point out.

"Anhydrous ammonia is a popular fertilizer, used directly by many farmers. It can also be used as a raw material to produce other kinds of nitrogen fertilizer — including, but not limited to, ammonium nitrate," our science team says.

[Note: Since we posted the 12:15 p.m. update above, Matt Cawthorn, chief deputy of the McLennan Co. Sheriff's Department, has told reporters that the situation remained volatile because ammonium nitrate had been found at the scene.]

UPDATE at 10:20 a.m. ET: President Obama Offers Condolences

"West is a town that many Texans hold near and dear to their hearts, and as residents continue to respond to this tragedy, they will have the support of the American people," President Obama said in a statement.

He said federal emergency agencies are in close contact with state and local officials in Texas and thanked first responders who worked through the night to contain the blaze.

UPDATE at 9:30 a.m. ET: Firefighters Among The Missing; Still In Search-And-Rescue Mode

Waco, Texas, police Sgt. William Patrick Swanton says the threat was "significantly less" than Wednesday night when a fire and explosion ripped through a fertilizer plant in the nearby city of West. Authorities have no new casualty figures, he says.

"I have heard five to 15 people and three to five firefighters, but none of that is verified," Swanton said at a morning news conference, adding that it was based on "very limited intel" and secondhand reports.

He said firefighters "are there on the ground still and still in what they call a search-and-rescue mode."

Authorities still have no word on what caused the fire, Swanton says.

Here's our original post:

The massive explosion Wednesday night at a fertilizer plant near Waco, Texas, killed an estimated 5 to 15 people, injured more than 160 others and devastated the town of West, officials said Thursday morning as they tried to piece together what happened.

There are fears that the death toll could be even higher.

West Mayor Tommy Muska, who warned Wednesday night that "there are a lot of people that will not be here tomorrow," said at a brief news conference early Thursday morning that "I ask for your prayers." The mayor had earlier described the explosion as being "like a nuclear bomb ... [a] big old mushroom cloud." The force of the explosion was picked up by the U.S. Geological Survey's earthquake monitors — it was the equivalent of a 2.1 magnitude temblor.

The cause of the blast, which as we reported Wednesday night happened as local firefighters were battling a blaze at the plant, had not yet been determined. Waco Police Sgt. William Patrick Swanton told reporters Thursday morning, "We're not indicating it was a crime, but we don't know. ... Until we know that it was an industrial accident, we will work it as a crime scene."

NPR's John Burnett, who got to the area of the explosion early Thursday, tells our Newscast Desk, "I spoke to one woman. Her son was playing football at the middle school there and he was lifted up in the air by the force of the explosion. They said they could see glass and debris flying through the air like shrapnel. They said it was the most terrifying experience of their lives." Burnett also spoke to Morning Edition.

The explosion destroyed or damaged dozens of homes, businesses and a nursing home.

Erick Perez, 21, of West was using his cellphone to record the scene from a distance as firefighters tried to put out the blaze. The Associated Press has posted a copy of that video here.

Note: As happens when stories such as this are developing, there will likely be reports that turn out to be mistaken. We will focus on news being reported by NPR, other news outlets with expertise, and statements from authorities who are in a position to know what's going on. And if some of that information turns out to be wrong, we'll update.